Single-use cameras have become very popular and, as a result, there is a need to increase the range of acceptable film exposure. Because this type of camera is very cost-sensitive, most camera manufacturers do not include an automatic means for controlling film exposure. Currently, most single-use cameras on the market rely on the latitude of the film to handle scenes that have different exposure needs. This can lead to poor quality pictures and dissatisfied customers. It has been proposed to use a liquid crystal filter to automatically control the film exposure in a camera. However, a liquid crystal filter uses polarizing filters in their construction. This places a severe limit on the maximum transmission of the filter. That limit will also reduce the performance of the camera in low light conditions.
U.S. Pat. No. 4,554,587 (the '587 patent) discloses a video camera including a pickup tube, a diaphragm mechanism and an electrochromic filter plate which are disposed along the optical path of incident image light on the pickup tube. When an illuminance of an object to be photographed is lower than a predetermined illuminance, only the diaphragm mechanism is used to suitably control an exposure of the pickup tube. When the illuminance of the object exceeds the predetermined illuminance level, the diaphragm mechanism is operated to keep its aperture size at a predetermined aperture size which is larger than an original minimum aperture size corresponding to a pinhole state. At the same time, the electrochromic filter plate continuously changes its light transmissivity in response to an aperture control signal, thereby controlling the exposure of the pickup tube.
A problem with the disclosure in the '587 patent is that an electrical power source, such as a battery, will be required to supply the voltage necessary to control the electrochromic filter plate 16. The necessity of a battery increases the expense of the disclosed apparatus and decreases the reliability (the battery may run out of power). A further problem with the '587 disclosure is the complexity of the control device which includes a preamplifier 20, a processing circuit, a smoothing circuit, two reference voltage generators, a differential amplifier 36, a voltage comparator 46 and an analog switch 44. Such complexity increases the cost and size of such a device.